For those that end up in my situation, here is some information that may become useful:

Just got a reply back from [email protected] after waiting 2 weeks.

I previously asked on this forum if I need to provide a statement with my application and I believe I was told that I didn't.

They replied:

If one is on "visitor" status and have special grounds to ask for
changing the status to dependent after arrival in Hong Kong, they may try to submit their applications to the Other Visas and Permits Section at 7th Floor, Immigration Tower, 7 Gloucester Road, Wan Chai . However, they will be required to furnish a written statement explaining the reason for not obtaining a visa before coming to Hong Kong. In addition, they are required to fill in the forms ID91, ID428A &ID520 and the documents as stated in those forms.

Those forms can be downloaded via internet :-

Each case will be considered on its own merits.

So I guess i need a statement. They also said except in the most unusual circumstances, are visitors allowed to change their immigration status after arrival.

I think it would be easier to apply for Dependancy visa outside of HK but would now be a costly exercise. I'm getting married in December 24 and not leaving back to OZ just to apply for this application.

Lucky I have plenty of evidence to show a de-facto relationship in Australia for over 2 years.

I now have to think of what to write. I have read the website and you will be automatically rejected if you talk about work or for a better future. The only reason you can write is to start and take up the natural niche within a family as a husband.